While 100% cotton fabrics provide excellent resistance to nuisance static created by friction rubbing at relative humidities above 45%, they generate considerable electric shocks when rubbed below 35% relative humidity. Fabrics made from blends of cotton and nylon have better durability than cotton fabrics but have antistatic properties as poor as 100% cotton fabrics at low relative humidity. It is known that nuisance static can be reduced to acceptable levels in fabrics by adding at least 2% by weight of fabric of fibers uniformly doped with carbon black. However, light colored fabrics cannot be produced by this method because of the streaks caused by the black antistatic fibers.
The use of bundles of continuous thermoplastic filaments containing a delustered sheath completely surrounding a carbon doped core of less than 10% by volume of the fiber is known to provide antistatic protection without streaks in light colored carpets or in continuous filament clothing in concentrations as low as 0.05% by weight. Similar single staple filaments blended with other staple fibers are known to provide antistatic protection in carpets with as little as 0.5% by weight of carpet.
In order for nylon and antistatic fibers to be intimately blended and spun into yarns with cotton, the nylon fibers and antistatic fibers must be cut into short length staple rather than used as continuous filaments. Since the yarn sizes commonly used in clothing are much smaller than those used in carpets, the linear density of the antistatic fibers must be much smaller, i.e. not exceed 6 decitex, in order to be able to provide antistatic protection at very low concentrations. This causes several problems not anticipated in the prior art. While fibers significantly larger than 6 decitex having a carbon doped core of less than 10%, e.g 4%, by volume have a gray color, fibers no greater than 6 decitex of this type are black because the sheath is no longer of sufficent thickness to hide the black core. Conductivity per unit length of fiber decreases as fiber diameter gets smaller and is further reduced with each break in continuity, such as occurs with fibers cut to the short length i.e less than 6.3 cm, needed to be spinnable on the cotton system or when fine yarns are stressed by wear or laundering. A higher weight percent of antistatic fibers are needed as yarn size decreases to achieve the same level of conductivity per length of yarn. This need for a higher weight percent of fibers along with a change to a black color more than offsets the smaller filament diameter, and can result in streaks in light colored antistatic clothing fabrics containing small sheath/core conductive fibers.
Conductive carbonaceous fibers small enough to be processible with textile fibers such as cotton are known to provide antistatic protection with a little as 0.09% by weight of fabric even if only placed in the warp. However, these fibers are large, at least 7 microns in diameter, and are therefore readily visible in light colored fabrics. Also, since carbonaceous fibers are produced by oxidation of organic fibers, they are brittle and become weaker as fiber size is reduced whereas carbon doped sheath/core thermoplastic fibers become stronger as the core becomes smaller.
It would be highly desirable to be able to use conductive sheath/core thermoplastic fibers in cotton/nylon blend fabrics in light shade clothing because the antistatic properties provided in this manner are permanent and do not wear out.